Introduction Mental health stigma causes a range of diverse and serious negative sequelae. Anti-stigma campaigns have largely aligned with medical theories and categorical approaches. Such campaigns have produced some improvements, but mental health stigma is still prevalent. The effect of alternative theoretical perspectives on mental health within anti-stigma campaigns has not been tested. Moreover, we do not know their effect on help-seeking intentions. Methods We conducted an online experimental pre-post study comparing the effects of two anti-stigma campaign posters on mental health stigma and help-seeking intentions. One poster adhered to the medical, categorical approach to mental health, whereas the other poster portrayed mental health problems in line with a non-categorical, continuous perspective. Results After controlling for familiarity with the campaign poster, country of residence and pre-test scores, we found no significant between-group differences in terms of help-seeking intentions and all stigma attitudes except for danger-related beliefs. That is, those who viewed the non-categorical poster reported an increased perception that people with mental health problems are dangerous. Discussion Our largely null findings may suggest the equivalence of these posters on stigma and help-seeking intentions but may also reflect the brevity of the intervention. Our findings concerning danger beliefs may reflect a Type I error, the complexities of stigma models, or the adverse effects of increased perceived contact. Further research is needed to test the effects of differing mental health paradigms on stigma and help-seeking intentions over a longer duration.